A liquid crystal display is being widely used as a most important display device in the multimedia society, including applications ranging from a cellular phone to a computer monitor, a laptop computer and a television set. In a liquid crystal display, many optical films are used so as to enhance display characteristics, and, among others, a retardation film plays a great role, for example, in improving the contrast when viewed from the front and oblique directions and the color tone compensation. As the conventional retardation film, a polycarbonate or a cyclic polyolefin is used, and all of these polymers are a polymer having a positive birefringence. Here, the “positive” and “negative” of the birefringence are defined as follows.
The optical anisotropy of a polymer film provided with molecular orientation by stretching or the like can be expressed by a refractive index ellipsoid shown in FIG. 1. Here, in the case of a stretched film, the refractive index in the fast axis direction in the film plane is denoted by nx, the refractive index in the film in-plane direction orthogonal thereto is denoted by ny, and the refractive index in the thickness direction of the film is denoted by nz. Incidentally, the fast axis indicates an axial direction in which the refractive index in the film plane is low.
The negative birefringence means that the stretching direction becomes the fast axis direction, and the positive birefringence means that the direction perpendicular to the stretching direction becomes the fast axis direction.
That is, uniaxial stretching of a polymer having negative birefringence results in a small refractive index in the stretching axis direction (fast axis: stretching direction), and uniaxial stretching of a polymer having positive birefringence results in a small refractive index in the axial direction perpendicular to the stretching axis (fast axis: direction perpendicular to the stretching direction).
Many of polymers have positive birefringence. The polymer having negative birefringence includes an acrylic resin and polystyrene, but the acrylic resin develops small retardation and shows insufficient properties as a retardation film. The polystyrene has: a problem with the retardation stability, for example, its large photoelastic coefficient in the low temperature region allows a change of the retardation with a slight stress; a problem in the optical properties, that is, the wavelength dependency of the retardation is high; and furthermore, a practical problem that the heat resistance is low. Therefore, this polymer is not used at present.
The wavelength dependency of the retardation as used herein means that the retardation varies depending on the measuring wavelength, and this can be expressed as a ratio R450/R550 of the retardation measured at a wavelength of 450 nm (R450) to the retardation measured at a wavelength of 550 nm (R550). In general, a polymer having an aromatic structure strongly tends to have a large value of R450/R550, leading to reduction in the contrast or viewing angle characteristics in a low wavelength region.
A stretched film of a polymer showing negative birefringence has a high refractive index in the film thickness direction and can be an unprecedented retardation film, and therefore, this film is useful as a retardation film for compensating the viewing angle characteristics of a display such as super-twisted nematic liquid crystal display (STN-LCD), vertical-alignment liquid crystal display (VA-LCD), in-plane switching liquid crystal display (IPS-LCD) and reflective liquid crystal display (reflective LCD), or as a film for compensating the viewing angle of a polarizing plate, and the demand on the market for a retardation film having negative birefringence is strong.
Methods for producing a film with a heightened refractive index in the thickness direction of the film by using a polymer compound having positive birefringence have been proposed. One of these is a treatment method including adhering a heat-shrinkable film to one or both surfaces of a polymer film and heat-stretching the laminate to apply a shrinkage force in the film thickness direction of the polymer film (see, for example, Patent Documents 1 to 3). Also, a method of uniaxially stretching a polymer film in an in-plane direction while applying an electric field thereto has been proposed (see, for example, Patent Document 4).
In addition, a retardation film composed of fine particles having negative optical anisotropy and a transparent polymer compound has been proposed (see, for example, Patent Document 5).
However, the methods proposed in Patent Documents 1 to 4 have a problem that the production process is very complicated, resulting in poor productivity. Also, control of, e.g., uniformity of the retardation is extremely difficult compared with the conventional control by stretching.
In the case of using a polycarbonate as the base film, its large photoelastic coefficient at room temperature allows a change in the retardation with a slight stress, giving rise to a problem with the retardation stability. This film also has a problem of large wavelength dependency of the retardation.
The retardation film obtained in Patent Document 5 is a retardation film imparted with negative birefringence by adding fine particles having negative optical anisotropy, and in view of simplification and profitability of the production method, a retardation film not requiring the addition of fine particles is demanded.
Furthermore, a fumaric acid diester-based resin and a film composed of the resin have been proposed (see, for example, Patent Documents 6 to 10).